A terahertz wave technology started to be most recently studied even in modern optical science and is a field having very high potential possibility. In particular, it is expected that the terahertz wave technology will be used as very important electromagnetic wave resources indispensable to future IT development. Also, the terahertz wave technology is regarded as a next generation technology that the whole world watches. The terahertz wave technology has a very wide range of applications and the significance of the terahertz wave technology is increasing.
Since the terahertz wave exhibits peculiar transmission and absorption characteristics according to properties of materials, the terahertz are variously usable in spectroscopy fields for material analysis. In order to apply the terahertz wave to the spectroscopy fields, a wide bandwidth and a high signal-to-noise ratio (SNR) are essential. As an inexpensive method satisfying such conditions, a photomixing technique for generating a continuous terahertz wave at room temperature is suitable.
A continuous terahertz wave generation technology using the photomixing technique is a method that inputs two lasers having slightly different frequencies to an opto-electronic (O/E) converter and generates an electromagnetic wave having a frequency corresponding to a beat frequency. The continuous terahertz wave generation technology is also referred to as an optical frequency down-converter. An opto-electronic converter, which is integrated with an antenna capable of radiating an electromagnetic wave, is used for frequency down-conversion. Such opto-electronic converters are made by a combination of various semiconductors. The opto-electronic converter operates on a similar principle to that of a photodetector and has a bandwidth of several terahertzs according to characteristics of a semiconductor used therein.
However, in general, phase noise exists in a photomixing-based continuous-wave terahertz generation and detection device, and the phase noise reduces an SNR of a continuous-wave terahertz detection system using phase information of signals, thus causing degradation of phase information detection performance. Major cause of main phase noise is phase noise generated in a continuous-wave laser light source. A laser light source having a narrow spectrum line width, that is, low phase noise characteristics, is used for reducing the phase noise of the photomixing-based continuous-wave terahertz generation and detection device. In the case of using a laser having a narrow spectrum line width, the phase noise itself of the laser is reduced and thus the fundamental phase noise problem can be solved. However, the complexity and cost of the laser increases, thus increasing the construction cost of the whole system. As a method of reducing the phase noise of the photomixing-based continuous-wave terahertz generation and detection device without using a laser having a narrow spectrum line width, there is a method of removing a phase delay difference between two optical signals used in photomixing, which is the cause of the phase noise, by adjusting lengths of optical lines through which the two optical signals pass. Since existing methods of accurately adjusting the lengths of two optical lines need not use a laser having a narrow spectrum line width, there is no great change in terms of costs of the system. However, since various optical components are used in two optical lines for optical signal splitting, optical frequency shift, or optical signal amplification, it is very inconvenient and difficult for a system manufacturer to accurately adjust the lengths of the optical lines by taking into account the phase delay characteristics thereof. As another problem, even when the lengths of the optical lines are adjusted to be equal to each other at any temperature, a temperature of a measurement system may be changed in the case of long-time measurement according to a change in a temperature of an ambient environment. Due to a change in phase characteristics according to a change in temperatures of various optical components existing in the optical lines for optical signal splitting, optical frequency shift, or optical signal amplification, a phase delay difference between two optical signals may occur, causing in an increase in the phase noise of the system. The phase noise of the system, which is generated by the phase delay difference, changes according to the temperature of the system.
As a method of reducing the phase noise of the system without using a laser having a narrow spectrum line width and without accurately adjusting the lengths of two optical lines, there is a method of reducing the phase noise by averaging a phase detection signal by using a lock-in amplifier. As a time constant increases, a phase noise reduction effect increases. However, in the case of the method of reducing the phase noise by increasing the time constant of the lock-in amplifier, a measurement time necessary for obtaining one phase response increases and thus this method is very difficult to use for high-speed measurement or 2D or 3D imaging applications.